The long play seems to be deliver retail stock trading at the lowest possible price. There's a lot of money to be made even if they don't try to maximize profits.

Plus, I bet they are banking on the existing brokerages not responding until things are too late for them. Most brokerages offer free trades and other perks, but limited to "high-value" customers. They won't drop trading fees until they absolutely have to, but by that time, their low-value customers will all be gone.